The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known engine control strategies to improve fuel economy and reduce fuel consumption in internal combustion engines include operating at a lean air/fuel ratio. This includes engines configured to operate in compression-ignition and lean-burn spark-ignition combustion modes. Engines operating at lean air/fuel ratios can have increased combustion temperatures leading to increased NOx emissions.
A known exhaust aftertreatment system and control strategy for managing and reducing NOx emissions includes a reductant injection control system and an associated reductant-selective catalytic reduction device. The reductant injection control system injects a reductant, e.g., urea into an exhaust gas feedstream upstream of an ammonia-selective catalytic reduction device to reduce NOx molecules to nitrogen and oxygen. Known ammonia-selective catalytic reduction devices decompose urea to ammonia, and the ammonia reacts with the NOx molecules in the presence of a catalyst to produce nitrogen. Some amount of ammonia can be stored on the ammonia-selective catalytic reduction device, enabling continued reduction of the NOx molecules when the urea injection control system is not capable of dispensing a controlled amount of urea.
Known control systems include dispensing reductant at a rate that corresponds to concentrations of engine-out NOx emissions to achieve NOx reduction without using excess amounts of reductant, i.e., dispensing reductant at a reductant/NOx stoichiometric ratio.